In the field of radio communications, there is an antenna diversity technique for improving a transmission characteristic by transmitting/receiving signals through a plurality of antennas. Such an antenna diversity technique has a transmission diversity in which signals are transmitted through a plurality of transmission antennas at a transmission side, and a reception diversity in which signals are received through a plurality of reception antennas at a reception side. The transmission diversity is a technique for improving the transmission characteristics at the reception side by forming a plurality of propagation paths, having low correlations with each other, between the transmission side and the reception side. The reception diversity is a technique for improving reception characteristics by selecting a signal from among a plurality of signals received through the reception antennas or by combining the plurality of signals. Particularly, in a transmission method which has a single reception antenna and uses only the transmission diversity for the transmission, the transmission characteristics can be improved without increasing a size or cost required for the antenna of a receiver to be disposed. Therefore, the aforementioned transmission method is effective for a cellular mobile communication system which imposes a greater restriction on a mobile terminal than a base station.
However, in radio transmissions which use the transmission diversity, a plurality of reception signals cancel each other out and disappear due to a phase relationship between the plurality of reception signals received at the reception side, thereby resulting in a problem that the transmission characteristics deteriorates. Note that in the following descriptions, a signal transmitted from a transmission antenna is referred to as a “transmission signal”, and the transmission signal received by a reception antenna via a propagation path is referred to as an “incoming signal”.
FIG. 21 is a diagram illustrating a phase relationship between two incoming signals A and B which are to be combined with each other at a reception end. As shown in (a) of FIG. 21, a received wave is formed by combining the incoming signal A with the incoming signal B so as to have a phase difference α therebetween. Therefore, when α=180 degrees (reversed phase) is satisfied, the incoming signal A and the incoming signal B cancel each other out, thereby causing the received wave to disappear ((b) of FIG. 21). Particularly, when a conventional phase modulation scheme in which a phase is constant during one symbol is used for the radio communications, the incoming signals cancel each other out during the entire symbol period, thereby eliminating a detection output. As a result, a transmission error occurs. FIG. 22 is a schematic diagram illustrating a phase relationship between the two incoming signals A and B obtained when the phase difference α=180 degrees is satisfied in the conventional phase modulation scheme.
Conventionally, as a transmission method which uses the transmission diversity based on the conventional phase modulation scheme, non-patent document 1 discloses a method in which transmission data is encoded by using time interleave and an error correction code so as to perform transmission by sweeping a phase of a signal to be transmitted from one of two transmission antennas, for example. FIG. 23 is a diagram illustrating a structure of a transmission system which uses the conventional transmission diversity disclosed in non-patent document 1. Note that a conventional QDPSK (Quaternary Differential Phase Shift Keying) is used as the modulation scheme.
In FIG. 23, a divider 301 distributes a transmission signal 310 transmitted from a transmitter 300 into two transmission signals 311 and 312, and then transmits the transmission signals 311 and 312 to two antenna blocks, respectively. An oscillator 303 generates a predetermined phase swept signal 313. A phase shifter 302 is provided in one of the two antenna blocks, and transmits, from an antenna 305, a transmission signal 314 generated by adding a transmission signal 312 to the phase swept signal 313. As a time function of the phase swept signal 313 to be added, a variation, having a length longer than a symbol length and shorter than a code length of the error correction code or a time interleave length, is selected. Note that the variation is asynchronous with a symbol period, and is different from a predetermined phase transition occurred during a time period corresponding to the symbol length, as disclosed in patent document 1 to be described later. Furthermore, in addition to the transmission diversity having the aforementioned structure, the transmission data is encoded by using the error correction code.
As described above, in the transmission method which uses the conventional transmission diversity disclosed in non-patent document 1, the transmission is performed by previously varying a phase of a signal transmitted from the transmission side, thereby shortening a time period, as shown in FIG. 22, during which a state where the incoming signals transmitted from the two transmission antennas cancel each other out, continues. Therefore, even if an error occurs, the error can be corrected by means of the time interleave and an error correction processing.
Further, in a modulation scheme, used in a transmission method disclosed in patent document 1, which is focused on a symbol waveform (phase waveform during a symbol), a phase of the symbol waveform synchronized with a symbol period T has a convex phase transition, and a detection output is obtained by performing delay detection, thereby eliminating a state where the detection output disappears due to a multipath. Furthermore, with an effect produced by combining the multipath, even the transmission characteristics can be improved. This improved effect is fundamentally effective when a delay amount τ of a delayed wave is within a predetermined range (0<τ<T).
FIG. 24 is a schematic diagram illustrating a phase transition of the symbol waveform disclosed in patent document 1. In the phase transition shown in FIG. 24, a transition width within a time length T (a symbol length) corresponding to one symbol is set up to a maximum phase transition amount φMAX, and a phase is changed in a parabola shape based on a function shown by the following formula (1).φ(t)=(4φMAX/T2)·t·(T−t);(0<t<T)  (1)
FIG. 25 is a diagram illustrating a structure of a transmission signal generation circuit 700 disclosed in patent document 1. As shown in FIG. 25, the transmission signal generation circuit 700 includes a differential encoding circuit 701, a waveform generation circuit 702, a quadrature modulator 704 and an oscillator 703. The transmission signal generation circuit 700 causes the differential encoding circuit 701 to differentially encode transmission data, the waveform generation circuit 702 to modulate the differentially encoded data by means of a symbol waveform having a convex phase redundancy, and the quadrature modulator 704 to convert the modulated data into a signal having a carrier wave frequency band.
Next, a phase relationship between incoming signals obtained when using a symbol waveform having such a convex phase redundancy will be described.
FIG. 26 is a schematic diagram illustrating a phase relationship between the two incoming signals A and B obtained when using the symbol waveform having the convex phase redundancy. In FIG. 26, it is assumed that the phase difference α is 180 degrees. In this case, even when there is a delay between the incoming signals, phases of the respective incoming signals are shifted in a convex manner. Therefore, within an effective interval (an interval in which correct received data can be obtained), while there is an interval in which the incoming signals cancel each other out and a received wave disappears (a point b of FIG. 26), there is also another interval in which the incoming signals do not cancel each other out and the received wave remains (a point a or c of FIG. 26). The incoming signals A and B are processed by means of a combination of delay detection and a low-pass filter, thereby making it possible to obtain an effective detection output. As a result, the transmission characteristics can be improved by obtaining a path diversity effect.
FIG. 27 is a schematic diagram illustrating a structure of a conventional transmission system which uses the transmission diversity based on a modulation scheme disclosed in patent document 1. As shown in FIG. 27, a delay unit 901 is provided among the transmission signal generation circuit 700, a first aerial 904 and a second aerial 905, thereby interposing a delay between signals to be transmitted from the first antenna 904 and the second antenna 905. In this case, a delay amount interposed between the signals to be transmitted is set such that the path diversity effect is well obtained. Thus, the transmission characteristic can be improved.    [patent document 1] patent application U.S. Pat. No. 2,506,747    [non-patent document 1] Akira Hiroike, “Combined Effects of Phase Sweeping Transmitter Diversity and Channel Coding”, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 41, NO. 2, May 1992, IEEE, USA, pages 170-176